This invention relates to an improved color-image diffusion process for use in conjunction with the operation of a multi-level, halftone, color-image output device. In particular, it relates to such a process which uniquely links (a) a special, early-stage, output-device-dependent color error vector diffusion practice respecting input color-image data and utilizing a halftone, output-device-dependent color palette which contains output-device-dependent output color values, with (b) a later-stage procedure for applying a dot-gain correction curve regarding the infeed intensity value of a pixel which is to be supplied to the output device in preparation for image outputting. While different multi-level, halftone, color-image output devices may be employed in the practice of the invention, a preferred embodiment of, and manner of practicing, the invention, are described herein in conjunction with employing a C,M,Y,K print engine, or printer, with respect to which practice of the present invention has been found to offer particular utility.
In accordance with the invention, color image source pixel data, converted to an appropriate input color space which is a matter of the user choice—the L,a*,b* color space being chosen for illustration purposes herein—is subjected to a unique color error diffusion process which involves another color-space conversion to an output color space, and which is based, at least in part, upon the vector color distance between the input color-space values of a color-diffusion-error-corrected pixel, and the closest findable possible device output color as represented in a specially constructed bi-tonal (halftone) color palette of values. This color palette is uniquely arrived at through the process of averaging (a) measured image output-device color output values, with (b) user-selected, idealized color output values, to create an employable color palette which, while truly device-dependent in fundamental nature, has been adjusted through the mentioned averaging procedure to result in the outputting of halftone printed output image colors that are especially pleasing to the observer.
Another unique feature of the diffusion practice of the present invention, with respect to utilizing the values created for the averaged and just-mentioned color palette, is that the selection of color-space values for a pixel is made in a special prioritized manner which give determining weight, as will be explained, to the quadrant-related arithmetic signages which are linked with the two-dimensional chrominance values that are associated with related predecessor (input) pixel chrominance values and successor (output) pixel chrominance values.
Here is an illustration regarding how one can envision this practice. This illustration is one chosen for convenience in describing this practice of the invention, though it should be understood that other color space choices may be employed if desired.
In this illustration, source image data resides in R, G, B color space, and from this space, source color-image pixel data is first converted to L,a*,b* color space—the so-called input color space—wherein a* and b* are chrominance values which are two-dimensional in nature, and which are associated with arithmetic (+/−) signage that places these values in different color-space “quadrants”. From an arithmetic signage point of view, these quadrants can be represented as: (+,+), (+,−), (−,+), and (−,−).
In the mentioned averaged color palette, L,a*,b* input color values are table-related to associated output C,M,Y, C+M, M+Y, C+Y, and C+M+Y output color values, When an input pixel's diffusion-corrected L,a*,b* values are color-distance compared with table L,a*,b* values for the purpose of selecting output values in the C,M,Y,K color space, a prioritizing algorithm is implemented in a manner which favors, first of all, a selection of C,M,Y,K values from that palette whose associated L,a*,b* values possess the same quadrant-associated chrominance arithmetic signage as that linked to the input pixel's chrominance signage.
Thus a (+, +) chrominance input pixel will result in priority selection, initially, of a C,M,Y,K output value whose associated a*,b* chrominance values reside in the (+, +) quadrant, a (−,+) chrominance input pixel will result in a C,M,Y,K output value having an association with the (−,+) quadrant; and so on. This approach, thus steers output selection to a particular limited range of choices in the palette, and results in a very pleasing color image output result from the employed output device.
With respect, therefore, to an output device such as the above-mentioned illustrative C,M,Y,K printer which will be discussed illustratively herein, the averaged color palette includes averaged values arrived at for the following colors, M,C,Y,C+M, M+Y, C+Y, and C+M+Y. Additionally, white and black (or C+M+Y, or K) values are useful to have in the palette, and so are effectively included. These white and black (or C+M+Y, or K) values are not specifically illustrated in the several color palettes shown in the drawings herein.
Following the event of priority-determined palette conversion of a color-diffusion-error-corrected pixel's values between the L,a*,b* and the C,M,Y,K color spaces, and before “submission” of a selected-value-set, halftoned output pixel to the color-image output device, an intensity correction, if needed, is applied through the use of an appropriately pre-created intensity correction curve, thus to minimize the problems associated with color-output dot gain.
The detailed description which follows below will more fully explain how color values intended to be sent to the output device are chosen using both the color-value palette and the practice of quadrant-based prioritizing just generally discussed.
As will become appreciated, the applications of these two uniquely linked stages of processing as just generally outlined (specialized and priority-weighted color error diffusion, and anti-dot-gain intensity correction) result in greatly improved and superior output device outputting of halftone color images. Very specifically, the stage of practice involving use of the mentioned color palette, and of arithmetic-signage quadrant weighting, coupled with the subsequent-stage process of dot-gain intensity correction, lead to a final output image which is extremely satisfactory.
These and other features and advantages which are attained by practice of the present invention will become more fully apparent as the detailed description which now follows is read in conjunction with the accompanying drawings.